Costa has claimed that Conte told him via text message at the end of the season that he no longer featured in his plans and has gone on to effectively replace the Spaniard with fellow countryman Alvaro Morata.
And now Costa's lawyer, Ricardo Cardoso, says the situation at Stamford Bridge has deteriorated to the point where his client will never play for the Blues again.
Furthermore, he has stated Costa will only consider a move to Atletico Madrid as the situation behind the scenes with the Premier League champions turns even uglier.
"We are going to activate all the legal mechanisms [to secure Costa's exit] and formally present a transfer request," he said in quotes translated by ESPN. "In three seasons at Chelsea he won two championships, and was the decisive players in those victories.
"When he was already with the Spain national team [for a friendly against Colombia last June] and before travelling to Brazil to enjoy his holidays he was released from Chelsea by SMS by the coach Antonio Conte.
"Obviously, this release, in the way it was done, and how it became known by everyone, was unfair treatment and a lack of respect to the player on the part of Conte which is unacceptable and inexcusable.
"Chelsea have not just not denied nor corrected the coach, either in public or private, but instead renewed his contract. This shows that they knew about and were in agreement with the release of Diego Costa and the way it was done. Such discriminatory behaviour makes it impossible for Costa to return to Chelsea while Antonio Conte is coach.
"There is no way that he can continue to play for Chelsea, a message which has been communicated various times to those in charge there. And he will not play for any other club that is not Atletico Madrid, a club where he had sporting success, but above all where he was always respected, even after he had joined Chelsea."
Costa has been allowed to miss Chelsea's return for pre-season and won't be part of their squad for the clash with Arsenal in the Community Shield on Sunday.
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